Compliant hydrodynamic gas bearings are ideally suited to conditions found in aircraft turbine engines since they are not subject to the operational and durability limitations characteristic of bearings which require complex lubricant, support, cooling, and sealing systems. However, there is a need for improvement in the control of spring rate of the foil support system to insure dynamic stability of the rotor-bearing system at all speeds.
One difficulty experienced in known compliant hydrodynamic gas bearings has been that the foils are required to be relatively stiff in order to achieve a desired spring rate. Unless the foil in known systems is stiff the bearing system exhibits an inability to control oscillatory motion between the movable and stationary members at certain critical bearing speeds resulting in reduced load-bearing capacity. In contradistinction, it is desirable to minimize stiffness of the foil in order to render it sufficiently compliant to conform uniformly to the shaft under all conditions.
Another problem relates to oscillation under load conditions. The shaft in a high-speed radial bearing tends to orbit about the geometric center of the bearing support and the amplitude of the oscillation is maximized at certain critical speeds. In order to control this oscillation, it is desirable to introduce friction into the system. Friction introduced into an air bearing system is of two types, namely, viscous damping and sliding, or Coulomb, friction. The relatively small viscous-damping forces are linearly related to velocity while the predominant friction forces are due to sliding, or Coulomb, friction. Thus, it is preferable to maximize Coulomb damping in the bearing assembly.